1. Field of the Invention
The present invention relates to a gradient coil apparatus for magnetic resonance imaging system (MRI system).
2. Prior Art
There has been a growing demand for higher speed in diagnosis and higher definition in diagnostic image. The gradient magnetic field performance has been improved in the ever-enhancing performance of the magnetic resonance imaging system.
The gradient magnetic field is generated in the form of pulses based on a given sequence. To increase the diagnostic speed, it is necessary to reduce the time for reaching the desired magnetic field (it is for turning on the gradient coil must be reduced. To cut down the rise time, a high voltage must be applied to the coil. Further, the insulation performance must be enhanced since a high voltage is produced among the gradient coils for X, Y and Z axes.
To enhance the precision of a diagnostic image, it is necessary to increase the magnetic field gradient. To increase the magnetic field gradient, it is necessary to increase the current to be applied. However, this causes an increase in the Joule heat of the gradient coil. Further, the coil configuration is getting increasingly complicated. To avoid local overheating, a magnetic resonance imaging system equipped with a gradient coil cooling apparatus is coming into general use.
As described above, increased coil voltage and increased Joule heat have become the salient features of a gradient coil apparatus. In the gradient coil apparatus, a plurality of coils have been molded into one integral structure with a solid insulation member. The heat exchanger for cooling the coil when required is also molded into one integral structure with the coils.
However, to enhance insulation performance to cope with the increasing coil voltage, the solid insulation member must be made thicker. To cope with the increasing Joule heat, the solid insulation member must be made thin in order to reduce the thermal resistance between the coil and heat exchanger. To be more specific, the improvement of the electric insulation performance is incompatible with the improvement of the cooling performance.
One of the techniques of improving the electrical insulation performance and cooling performance is disclosed in the Patent Document 1, wherein a gradient coil is immersed in insulation oil and insulation oil is subjected to controlled circulation using a heat exchanger located outside the gradient coil, whereby improvement of both the electric insulation performance and cooling performance is achieved.
[Patent Document 1] Japanese Application Patent Laid-open Publication No. Hei 11-225983